Question: Factor the following expression: $-2$ $x^2$ $-1$ $x+$ $28$
Explanation: This expression is in the form ${A}x^2 + {B}x + {C}$ . You can factor it by grouping. First, find two values, $a$ and $b$ , so: $ \begin{eqnarray} {ab} &=& {A}{C} \\ {a} + {b} &=& {B} \end{eqnarray} $ In this case: $ \begin{eqnarray} {ab} &=& {(-2)}{(28)} &=& -56 \\ {a} + {b} &=& & & {-1} \end{eqnarray} $ In order to find ${a}$ and ${b}$ , list out the factors of $-56$ and add them together. Remember, since $-56$ is negative, one of the factors must be negative. The factors that add up to ${-1}$ will be your ${a}$ and ${b}$ When ${a}$ is ${7}$ and ${b}$ is ${-8}$ $ \begin{eqnarray} {ab} &=& ({7})({-8}) &=& -56 \\ {a} + {b} &=& {7} + {-8} &=& -1 \end{eqnarray} $ Next, rewrite the expression as ${A}x^2 + {a}x + {b}x + {C}$ $ {-2}x^2 +{7}x {-8}x +{28} $ Group the terms so that there is a common factor in each group: $ ({-2}x^2 +{7}x) + ({-8}x +{28}) $ Factor out the common factors: $ x(-2x + 7) + 4(-2x + 7) $ Notice how $(-2x + 7)$ has become a common factor. Factor this out to find the answer. $(-2x + 7)(x + 4)$